Low-frequency sound affects active micromechanics in the human inner ear

Noise-induced hearing loss is one of the most common auditory pathologies, resulting from overstimulation of the human cochlea, an exquisitely sensitive micromechanical device. At very low frequencies (<250 Hz), however, the sensitivity of human hearing, and therefore the perceived loudness is poor. The perceived loudness is mediated by the inner hair cells of the cochlea, which are driven very inadequately at low frequencies. To assess the impact of low-frequency sound, we exploited a by-product of the active amplification of sound outer hair cells perform, so-called spontaneous otoacoustic emissions. These are faint sounds produced by the inner ear that can be used to detect changes of cochlear physiology. We show that a short exposure to perceptually unobtrusive, low-frequency sounds significantly affects outer hair cells: A 90 s, 80 dB(A) low-frequency sound induced slow, concordant and positively correlated frequency- and level oscillations of spontaneous otoacoustic emissions which lasted for about two minutes after low-frequency sound offset. Low-frequency sounds, contrary to their unobtrusive perception, strongly stimulate the human cochlea and affect amplification processes in the most sensitive and important frequency range of human hearing.

噪声性听力损失（Noise-induced hearing loss）是最为常见的听觉病理之一，其成因是人类耳蜗（cochlea）受到过度刺激——耳蜗本身是一种极为精密灵敏的微机械装置。然而在极低频（<250 Hz）环境中，人类听觉的灵敏度乃至主观感知响度均表现较差。主观感知响度由耳蜗内毛细胞介导，而这类细胞在低频环境下的激活程度极低。
为评估低频声音的影响，我们利用了外毛细胞主动放大声音时产生的副产物，即所谓的自发性耳声发射（spontaneous otoacoustic emissions）。这类由内耳自发产生的微弱声波，可用于检测耳蜗生理功能的变化。
研究结果显示，短时间暴露于感知上难以察觉的低频声音，会对外毛细胞产生显著影响：暴露时长90秒、声压级为80 dB(A)的低频声音，可诱使自发性耳声发射出现缓慢、协调且呈正相关的频率与振幅振荡，该振荡状态会在低频声音结束后持续约两分钟。
与人们主观感知上的不明显性相反，低频声音会强烈刺激人类耳蜗，并对人类听觉最灵敏、最为关键的频率范围内的声音放大过程产生影响。